Multilevel converters, particularly Neutral-Point-Clamped Converters (NPCC), are widely used for single- and three-phase bi-directional grid-connected applications in the medium voltage (MV) range, including rolling mills, fans, pumps, marine appliances, mining, traction, uninterrupted power supply (UPS) and renewable energy integration. Attractive characteristics of multilevel converters include relatively low harmonic amplitudes at the converter input and output terminals, reduced switching losses, as well as reduced electromagnetic interference. On the other hand, the number of switches connected in series increases along with the number of converter levels. Accordingly, the total converter cost increases and so do the semiconductor conduction losses. Multilevel converter topologies therefore are most suitable for medium to high voltage and high power applications for which no high-voltage blocking semiconductors are available to date.
In order to reduce the cost of power electronic converters, circuit topologies with low semiconductor part counts are favoured, which also may improve both the system power density (low weight converter) and power efficiency. An exemplary four-level Active Neutral-Point-Clamped Converter (ANPCC) circuit topology with a reduced number of diodes is disclosed in WO 2011/132206 A1.
EP 0 727 870 A2 and EP 0 481 304 A2 relate to multi-level converters with converter branches comprising several controllable switches connected in series.
CN 102 769 404 A shows several converter bridges, each of which comprises four converter branches with one switch, which do not have intermediate connections with each other.